In recent years, a semiconductor light-emitting device employing a semiconductor light-emitting element such as a light-emitting diode (LED) has been developed as a light source for illumination. In particular, a white semiconductor light-emitting device implementing white light by wavelength-converting light emitted from an LED by a phosphor or the like is expected as a light-emitting device substituting for a fluorescent lamp.
However, the light emitted from the LED or the like employed in such a semiconductor light-emitting device has strong directivity, and hence the light output may have remarkably varied with the angle of observation. In the aforementioned white semiconductor light-emitting device employing the wavelength conversion technique, there has been such an inconvenience that the color tone changes in response to the strength/weakness of the light output from the LED and hence the color tone varies with the angle of observation.
As a related technique, there is a method of reducing the directivity of light emitted from an LED by arranging a diffusion material around the LED (refer to Japanese Patent Laying-Open No. 2002-50797, for example).
As shown in FIG. 13, an LED 101 of ultraviolet emission is fixed to the bottom surface of a recess portion of a metal frame 102 with solder or the like (not shown) in an LED lamp 100 of the related technique. Thus, one electrode (not shown) of the LED 101 is electrically connected with the frame 102. One end of a first lead 103 is mounted on the frame 102. Another electrode (not shown) of the LED 101 is electrically connected with one end of a second lead 105 through a metal wire 104.
A light diffusion portion 106 covering the LED 101 arranged on the bottom surface side and a phosphor portion 107 arranged on the light diffusion portion 106 are formed in the recess portion of the frame 102. The light diffusion portion 106 is made of silicon resin containing a diffusion agent consisting of alumina particles or the like. The phosphor portion 107 is made of silicon resin containing three types of phosphors (hereinafter abbreviated as RGB phosphors) of an ultraviolet excitation red emission phosphor (R phosphor), an ultraviolet excitation green emission phosphor (G phosphor) and an ultraviolet excitation blue emission phosphor (B phosphor).
Further, a shell-like sealing portion 109 of silicon resin is so formed as to seal the frame 102 in which the LED 101 is set, the wire 104 and the joint between the wire 104 and the second lead 105. Other ends of the first lead 103 and the second lead 105 are exposed from the sealing portion 109. Thus, the LED lamp 100 of the aforementioned related technique is constituted.
In this LED lamp 100, ultraviolet light emitted from the LED 101 is diffused by the light diffusion portion 106. Thereafter the diffused ultraviolet light is converted to red, green and blue by exciting the RGB phosphors in the phosphor portion 107 respectively. Then, these are so mixed with each other that white light can be implemented.
In the LED lamp 100 according to the aforementioned related technique, however, the diffusion agent consisting of alumina particles or the like is mixed into the silicon resin in a prescribed compounding ratio, and a substance obtained by stirring this is charged into the recess portion of the frame 102 thereby forming the light diffusion portion 106. At this time, it is difficult to homogeneously mix the silicon resin and the diffusion agent with each other, and hence distribution of the diffusion agent in the light diffusion portion 106 easily becomes heterogeneous. Therefore, the most part of the light emitted from the LED 101 may be blocked by the diffusion agent, or the emitted light may be hardly diffused but transmitted. Consequently, the LED lamp 100 according to the aforementioned related technique has such a problem that the light intensity and the color tone easily vary with the direction of observation.
In the LED lamp 100 according to the aforementioned related technique, the thickness of the light diffusion portion 106 or the concentration of the diffusion agent in the light diffusion portion 106 is increased, in order to solve the aforementioned problem. In this case, however, there is such a problem that the ratio of light absorbed by the light diffusion portion 106 is increased following increase in the number of reflection times in the light diffusion portion 106 and hence an outwardly radiated light output is reduced.